Tape printers are well-known for unwinding and running wound long tape by a direct current motor (designated as DC motor hereinafter) and performing line printing on the unwound tape by using dot patterns. Some of such tape printers are provided with a cutter for cutting the tape downstream from the printing position. A printed strip having a desired length is produced because the original tape can be cut manually or automatically after printing.
The above tape printers may be provided with an encoder to detect an amount of rotation of the DC motor. The encoder has a rotary disk having radial slits formed peripherally at regular intervals. The disk is connected to an output shaft of the DC motor. The encoder has a photo-sensor having a light-emitting element and a light-receiving element disposed at the opposite sides of the rotary disk. When the encoder is used, the printing head is driven to print on the tape, every time the amount of rotation of the DC motor increases by a predetermined amount based on an output pulse signal of the light-receiving element. This arrangement allows the tape to be constantly printed at uniform dot intervals in the running direction of the tape, regardless if the tape runs at a constant speed or not.
In the tape printer provided with the above cutter, the printing position of the printing head such as a thermal head is inevitably separated from the cutting position of the cutter by a certain distance due to the structure of the printing machine. Then, the printed tape made at the start of each printing inevitably has a leading margin on one end whose length is equal to the distance between the printing position and the cutting position. As a result, the user has to manually cut the leading margin after the printing is over, which may lead to inconvenience for the user. In order to obtain a printed strip having less leading margin, the DC motor is suspended during the printing. The tape is cut while the movement of the tape is suspended. And then, the DC motor is required to be activated again to resume the printing. Additionally, if the volume of image data (including character data and graphic data) for the printing exceeds the memory capacity of the tape printer, the following operation is required: the image data stored in the memory first is printing and then the rotation of the DC motor is stopped. During the stop of the DC motor, the remaining image data is entered into the memory from an external device. After the DC motor starts rotating again, the printing is resumed.
When the power supply to the DC motor is suspended in order to temporarily stop the running tape, the number of rotations of the DC motor gradually decreases so that the DC motor finally stop. Accordingly, the tape stops running. However, the DC motor may be subject to a resilient force of the tape conveying mechanism which is produced by the release of the forward deflection of the platen roller accumulated by power supply, which may result in reverse rotation of the DC motor. On the other hand, since the tape is firmly pinched by the platen roller and the printing head, the tape is hardly moved backward even if the DC motor rotates reverse. Therefore, if the DC motor rotates reverse, a certain period of time is necessary until the DC motor restores the original position when the power supply to the DC motor is resumed. Accordingly, Discrepancy may be caused between the time when the power supply to the DC motor is resumed and the time when the tape actually runs again. Then, the dots printed after the suspension of the running tape may not be properly connected to the dots printed before the suspension in the tape running direction, which may affect the printing quality.
An object of the present invention is to provide a tape printer that prevents displacement of printed dots in a running direction of the tape due to reverse rotation of the DC motor caused by the suspension.